1. Field of the Invention
The present invention relates in general to an organic electroluminescent device (OELD) and a display incorporating the same, and more particularly, to an organic electroluminescent device whose salt in the electron source has a concentration with a spatial distribution and a display incorporating the same.
2. Description of the Related Art
With respect to the development of an organic electroluminescent device (OELD), the material of the cathode must be a low work function metal with high activity, so in early stage, the metal alloy of magnesium and less silver substantially in the ratio of 10:1 is used as an excellent cathode alloy. However, a highly active metal is not applicable to the OELD manufacturing process, so an inactive metal, such as aluminum (Al) for instance, is used as an electron injection electrode. However, the work function of the inactive metal can not match with the energy level of the lowest unoccupied molecular orbital (LUMO) of an organic layer. In practical application, there is a need to insert a thin layer of salt, such as lithium fluoride (LiF) for instance, between aluminum (Al) and the organic layer. Thus, tunneling effect is activated due to the insulating characteristic of LiF, so that the electron injection efficiency is largely enhanced, and that the unmatching problem of the work function of inactive metal is resolved.
With regard to the development of electron transport layer, the most effective practice is to dope a highly active metal such as lithium (Li) or cesium (Cs) to an organic electron transport material, so that radical anions and charge transfer (CT) complexes can be formed. Therefore, the operating voltages of the OELD are reduced significantly. The conductivity of the metal organic thin film doped with lithium (Li) is about 3*10−5 (S/cm), which means the inside carrier density can be as high as 1018 (cm−3). Since an alkali metal (I A) or an alkaline metal (II A) such as cesium (Cs), lithium (Li), and magnesium (Mg) has high activity and is very sensitive in terms of metal doping, a small amount of deviation will lead to a significant change in luminance efficiency and operating lifespan of the OELD. Therefore, the alkali metal (I A) and the alkaline metal (II A) are not applicable to the heat evaporation manufacturing process during the manufacturing process of OELD. Moreover, the metallic ions of a doped layer, when driven by a current, will be spread over and activate luminance quenching effect in the organic emissive layer, causing the luminance efficiency and operating lifespan of OELD to be reduced.